J. E. A. Marshall

Are we now living in the Anthropocene

The term Anthropocene, proposed and increasingly employed to denote the current interval of anthropogenic global environmental change, may be discussed on stratigraphic grounds. A case can be made for its consideration as a formal epoch in that, since the start of the Industrial Revolution, Earth has endured changes sufficient to leave a global stratigraphic signature distinct from that of the Holocene or of previous Pleistocene interglacial phases, encompassing novel biotic, sedimentary, and geochemical change. These changes, although likely only in their initial phases, are sufficiently distinct and robustly established for suggestions of a Holocene–Anthropocene boundary in the recent historical past to be geologically reasonable. The boundary may be defined either via Global Stratigraphic Section and Point (“golden spike”) locations or by adopting a numerical date. Formal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions. This datum, from the perspective of the far future, will most probably approximate a distinctive stratigraphic boundary.

Stratigraphy of the Anthropocene

The Anthropocene, an informal term used to signal the impact of collective human activity on biological, physical and chemical processes on the Earth system, is assessed using stratigraphic criteria. It is complex in time, space and process, and may be considered in terms of the scale, relative timing, duration and novelty of its various phenomena. The lithostratigraphic signal includes both direct components, such as urban constructions and man-made deposits, and indirect ones, such as sediment flux changes. Already widespread, these are producing a significant ‘event layer’, locally with considerable long-term preservation potential. Chemostratigraphic signals include new organic compounds, but are likely to be dominated by the effects of CO2 release, particularly via acidification in the marine realm, and man-made radionuclides. The sequence stratigraphic signal is negligible to date, but may become geologically significant over centennial/millennial time scales. The rapidly growing biostratigraphic signal includes geologically novel aspects (the scale of globally transferred species) and geologically will have permanent effects.

Constraints on the numerical age of the Paleocene‐Eocene boundary

Here we present combined radio-isotopic dating (U-Pb zircon) and cyclostratigraphic analysis of the carbon isotope excursion at the Paleocene/Eocene (P/E) boundary in Spitsbergen, to determine the numerical age of the boundary. Incorporating the total uncertainty from both radio-isotopic and cyclostratigraphic datasets gives an age ranging from 55.728-55.964 Ma, within error of a recently proposed astronomical age of ~55.93 Ma. Combined with the assumption that the Paleocene Epoch spans twenty-five 405 kyr cycles, our new age for the boundary suggests an age of ~66 Ma for the Cretaceous/Paleogene (K/Pg) boundary. Furthermore, our P/E boundary age is consistent with the hypothesis that the onset of the Paleocene-Eocene thermal maximum (PETM) at the boundary occurred on the falling limb of a 405 kyr cycle, suggesting the event was initiated by a different mechanism to that which triggered the other early Eocene hyperthermals.

Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset, UK

For the purposes of a high-resolution multi-disciplinary study of the Upper Jurassic Kimmeridge Clay Formation, two boreholes were drilled at Swanworth Quarry and one at Metherhills, south Dorset, UK. Together, the cores represent the first complete section through the entire formation close to the type section. We present graphic logs that record the stratigraphy of the cores, and outline the complementary geophysical and analytical data sets (gamma ray, magnetic sus- ceptibility, total organic carbon, carbonate, δ 13 C org ). Of particular note are the new borehole data from the lowermost part of the formation which does not crop out in the type area. Detailed logs are available for download from the Kimmeridge Drilling Project web-site at http://kimmeridge. earth.ox.ac.uk/. Of further interest is a mid-eudoxus Zone positive shift in the δ13C org record, a feature that is also registered in Tethyan carbonate successions, suggesting that it is a regional event and may therefore be useful for correlation. The lithostratigraphy of the cores has been precisely correlated with the nearby cliff section, which has also been examined and re-described. Magnetic-susceptibility and spectral gamma-ray measurements were made at a regular spacing through the succession, and facili- tate core-to-exposure correlation. The strata of the exposure and core have been subdivided into four main mudrock lithological types: (a) medium-dark-dark-grey marl; (b) medium-dark-dark grey-greenish black shale; (c) dark-grey-olive-black laminated shale; (d) greyish-black-brownish- black mudstone. The sections also contain subordinate amounts of siltstone, limestone and dolostone. Comparison of the type section with the cores reveals slight lithological variation and notable thick- ness differences between the coeval strata. The proximity of the boreholes and different parts of the type section to the Purbeck-Isle of Wight Disturbance is proposed as a likely control on the thickness changes.

Earliest Carboniferous tetrapod and arthropod faunas from Scotland populate Romer's Gap

Devonian tetrapods (limbed vertebrates), known from an increasingly large number of localities, have been shown to be mainly aquatic with many primitive features. In contrast, the post-Devonian record is marked by an Early Mississippian temporal gap ranging from the earliest Carboniferous (Tournaisian and early Viséan) to the mid-Viséan. By the mid-Viséan, tetrapods had become effectively terrestrial as attested by the presence of stem amniotes, developed an essentially modern aspect, and given rise to the crown group. Up to now, only two localities have yielded tetrapod specimens from the Tournaisian stage: one in Scotland with a single articulated skeleton and one in Nova Scotia with isolated bones, many of uncertain identity. We announce a series of discoveries of Tournaisian-age localities in Scotland that have yielded a wealth of new tetrapod and arthropod fossils. These include both terrestrial and aquatic forms and new taxa. We conclude that the gap in the fossil record has been an artifact of collection failure.

The age, fauna and palaeoenvironment of the Late Triassic fissure deposits of Tytherington, South Gloucestershire, UK

Important vertebrate faunas occur in fissure deposits of Late Triassic–Jurassic age in SW Britain. Although the faunas are well described, their age and palaeoenvironment remain poorly understood. One such fissure system was documented in detail during quarrying operations at Tytherington and has yielded in situ palynomorphs that add much information concerning its age and palaeoenvironment. Significantly, the Tytherington fauna is of the sauropsid type that has generally been dated as Norian or pre-Penarth Group transgression and was also regarded as representing a distinct upland fauna. The palynomorphs, which include a significant marine component, demonstrate that the Tytherington Triassic fissures are infilled with Late Triassic (Rhaetian) sediments that match specific levels in the Westbury Formation. In addition, many of the Tytherington solutional fissures probably formed during the Rhaetian and are consistent with a fluctuating saline to freshwater environment. There is no prima facie evidence of solutional formation and infilling of the reptile-bearing deposits before the Rhaetian trangression. The fissure reptile fauna, which includes the early dinosaur Thecodontosaurus , inhabited a small fire-swept limestone island in the Rhaetian sea. The features of the herpetofauna are entirely consistent with this island model which has Quaternary analogues.

Chitinozoan reflectance : a Lower Palaeozoic thermal maturity indicator

Abstract Chitinozoan reflectance ( R ch ) is an innovative technique of thermal maturation determination suitable for Lower Palaeozoic sediments where the established reflectance scale, vitrinite reflectance ( R v ), is not applicable due to the lack of vitrinite. Reflectance measurements are taken from polished sections of chitinozoa, a group of Lower Palaeozoic, organic walled marine microfossils. These vase-shaped palynomorphs are optically isotropic, have thick, homogeneous test walls and are easy to identify, making then ideal for organic petrology. From over 1000 samples collected, 54 possessed both chitinozoa and vitrinite, enabling direct calibration of the two scales. A clear linear relationship exists between the two parameters. The equation resulting from this study is: R ch = 1.152 R v +0.08, with a correlation coefficient of 0.989. There is a suggestion of a maturity discontinuity between 1 and 2% R ch , but due to the high sample redundancy rate (only 5% of samples possessed both vitrinite and chitinozoa), it was not possible to supply more data at this maturity level. Chitinozoan reflectance is a powerful thermal maturity indicator for Lower Palaeozoic sediments. The technique has significant advantages over its maln rivals, particularly the ease of particle identification and the fact that it is independent of facies. Thus, quantitative maturity data with a high spatial and temporal resolution can be gained from pre-Devonian marine sedimentary sequences.

Palynology of the Stonehaven Group, Scotland; evidence for a Mid-Silurian age and its geological implications

Palynomorphs from the Stonehaven Group indicate a late Wenlock to early Ludlow age. This is older than the currently accepted Pridoli age based on fish and arthropods. Reasons for this discrepancy are discussed. A previous correlative of the Stonehaven Group has been the Old Red Sandstone of Kerrera, Argyll. However, palynological assemblages from both Kerrera and the adjacent succession at Oban are in fact younger than that from Stonehaven in being of latest Silurian to earliest Devonian age. The palynomorphs from Kerra and Oban lie beneath the important geochronological tie-point of the Lorne Lavas and suggest that the Silurian-Devonian boundary is older than currently accepted.

Organic maturation, thermal history and hydrocarbon generation in the Orcadian Basin, Scotland

The Devonian Orcadian Basin contains thick sequences of organic-rich lacustrine sediments of interest as hydrocarbon source rocks. Throughout the basin the range of organic maturation is large, spore colours vary from yellow-orange to black, and vitrinite reflectance for Type III kerogens ranges from 0.7 to 10.5%. Maximum palaeotemperatures are estimated to range from those typical of moderate burial diagenesis (100°C) to those characteristic of greenschist facies metamorphism (400°C). The highest maturities are related to contact metamorphism by Devonian plutons, and it is postulated that a large pluton underlies much of Caithness. High and low maturity regions are often juxtaposed across major faults which were active during Variscan inversion. Regionally, the lowest maturity is about 0.8% Ro, implying uplift and erosion of 2–3 km of cover, basin wide. This cover was probably a sequence of Late Devonian and Carboniferous rocks removed largely during Variscan uplift. Geothermal gradients were high but variable during Devonian extension. This, together with the contact metamorphic affects of the Devonian plutons, resulted in early maturation/metamorphism of large areas of the basin. Most of the onshore sequence entered the oil window during Devonian and Carboniferous times. Significant generation of hydrocarbons from Orcadian lacustrine facies offshore during subsequent Mesozoic burial will have occurred only in those areas which escaped maturation during the Palaeozoic.

Rhabdosporites langii, Geminospora lemurata and Contagisporites optivus: an origin for heterospory within the progymnosperms

A major mid-Devonian floral event is the incoming and proliferation of the archaeopteridalean progymnosperms. They are generally accepted to have originated from the aneurophytalean progymnosperms. The spores from both groups (the microspore Geminospora lemurata and its megaspore Contagisporites optivus—Archaeopteridales; Rhabdosporites langii—Aneurophytales) are well represented in the Orcadian Basin, Scotland. Study of a long section in southern Orkney shows that G. lemurata first occurs not by progressive size reduction of R. langii but as a small thick-walled spore (“early form”) which is presumed to originate by heterochrony directly from an immature Rhabdosporites. It then progressively increases in size. This early stage of evolution can be timed using the lacustrine cyclicity. Subsequently a rapid increase in abundance occurs with G. lemurata replacing R. langii as the dominant element of the palynofloras. Originating after the first appearance of G. lemurata are spores transitional in morphology between R. langii and C. optivus indicating the progressive development of the megaspore. These changes together with the presence of a three-walled species of Rhabdosporites (Rhabdosporites streelii Marshall, sp. nov.) give a hint of the complex changes occurring within the sporangia of the archaeopteridalean group at this time. These changes show how detailed investigation of related spore taxa can reveal the pattern of the development of heterospory in the Progymnosperms. This permits for the first time a test of the theoretical models for the development of heterospory. The recognition that an “early form” of G. lemurata exists indicates that some caution should be exercised in using the first occurrence of this species as a stratigraphic marker. Rhabdosporites parvulus is confirmed as a small R. langii but the bulk of specimens formerly attributed to this species are G. lemurata. The means that the Eday Group sediments of Orkney are contemporaneous with the Devonian deposits of southeast Shetland and Fair Isle.